In an effort to gain more control over fuel injection timing, quantity and rate shaping, a class of fuel injectors has evolved with the ability to directly control the position of the needle valve that opens and closes the nozzle outlet. In many of these types of fuel injectors, the needle valve member includes a closing hydraulic surface exposed to fluid pressure in a needle control chamber. A separate needle control valve has the ability to expose the needle control chamber to either low or high pressure, independent of fuel injection pressure and engine crank shaft position. This permits the injection quantity and timing to be precisely controlled and optimized based upon sensed operating conditions, such as engine load and speed, etc. This control permits engineers to provide an improved balance between undesirable exhaust emissions and engine performance across an engine's operating range. However, there remains room for improvement, particularly in the area of rate shaping each injection event to further improve combustion efficiency at various engine operating conditions.
Engineers have come to recognize that undesirable exhaust emissions can often be reduced if each injection event ends as abruptly as possible. Many fuel injectors with directly controlled needle valves can accomplish abrupt needle closure since they have the ability to close the needle valve even while fuel injection pressures are relatively high. On the front side of each injection vent, engineers have observed that undesirable exhaust emissions can often be reduced over much of an engine's operating range if the injection event begins more gradually. Since gradual beginning and abrupt ending to injection events often places contradictory constraints on the workings of an individual fuel injector, there remains room to improve the rate shaping characteristics of fuel injectors with direct control needle valves.
The present invention is directed to overcoming these and other problems associated with rate shaping in fuel injectors having direct control needle valves.